Ultrafast lasers control metallic electrons

GARCHING, Germany – A technique
that controls the motion of electrons using very fast laser pulses, first demonstrated
using gaseous atoms or molecules, has been shown for the first time to work for
electrons emitted from a solid metal tip. The results could allow scientists to
study electron dynamics in solid-state systems on subfemtosecond and subnanometer
scales; the simple, compact and very sensitive system could lead to the development
of ultrafast optical transistors.

Typical time structure of the electric field of femtosecond laser
pulses. The maximum of the oscillation of the carrier wave (blue) depends upon the
phase relative to the maximum of “the envelope” (green). In the left
pulse, the phase difference amounts to 180°, whereas in the right pulse, 0°.
Courtesy of MPQ.
Researchers at the Max Planck Institute of Quantum Optics (MPQ)
showed that they can steer the emission of electrons from the metal tip with the
phase of the optical cycle using relatively small laser intensities.

The researchers used a tungsten tip that was irradiated with light
pulses a few femtoseconds in duration and discovered that, if the intensity of the
pulse is high enough, the electrons could absorb the amount of energy needed to
be released from the metal tip. With a curvature radius of about 10 nm, the extremely
sharp tip greatly amplifies the intensity of the laser light. Because of their short
duration, the laser pulses contain only a few cycles.

Energy spectra of electrons for different phase shifts (180° and 0°). At a phase shift
of 180°, pronounced equidistant maxima are observed, whereas there is no interference
structure for 0°. The insets explain the physical processes: On the left, electrons
with high energies are emitted at two time intervals (red ellipse) during the pulse,
leading to the quantum mechanical interference pattern of the spectrum. Instead,
on the right, electron emission is possible only once per pulse; therefore, no interference
can occur. In this case, however, the electrons gain on average more kinetic energy,
and the number of electrons at higher energies is larger. The shallow slope of the
curve between 5 and 10 eV (especially at 0°) indicates elastic re-scattering
in the laser field. Courtesy of MPQ.
During the experiment, the scientists measured the kinetic energy
of the emitted electrons for different phase shifts. They noted that the structure
of the electron spectrum was strongly influenced by the phase shift.

“The higher the electron energy, the more we approach the
situation that we are able to switch the current on or off by simply changing the
phase shift by 180 degrees,” said Michael Krüger, a co-author of the
paper published in the July 6 issue of Nature (doi: 10.1038/nature10196).

The researchers also observed that the chosen phase shifts determined
whether pronounced peaks in the spectra could be observed. These maxima are a consequence
of the quantum mechanical wave nature of electrons. Electrons could be emitted during
two time intervals of the light pulse at a phase shift of 180°, and the interference
of the two matter wave packets at the metal tip lead to the observed interference
in the spectrum. On the other hand, if the phase shift equals 0°, the electrons
would be emitted only once per pulse, making the maxima disappear, and no interference
would occur.

Metal tip irradiated with a laser pulse. Courtesy of Thorsten Naeser, MPQ.
The researchers concluded that the laser field would continue
to influence the motion of the electrons even after emissions from the metal tip
occurred. It is implied that the released electrons would be driven back into the
tip by the laser field and scatter elastically off the tip before being detected.

“As is demonstrated in the experiment, this scattering process
does not destroy the interference of the electron wave packets, that means it takes
place in a coherent way,” concluded co-author Markus Schenk.